Chromeless phase shift lithography (CPL) has been investigated for many years as a possible single-mask resolution enhancement technique for lines/spaces in semiconductor devices. For positive resists, it is particularly well-suited to the patterning of semi-isolated narrow lines but not to dense line/spaces or contacts. However, with significant mask design effort and added mask complexity, contacts and semi-dense line/spaces have been successfully patterned. Like other phase shifting techniques, such as alternating PSM (phase shift mask), CPL can provide significantly better aerial image contrast compared to binary masks; unlike alternating PSM, however, it is a single mask single exposure technique avoiding many of the dual-reticle concerns such as throughput, mask layout, and reticle-to-reticle overlay.
CPL uses phase edges between 0° and 180° phase shift regions on the mask to pattern lines along the phase edges. This is possible without chrome because destructive interference of light diffracted from regions immediately on either side of the phase edge result in an aerial image minimum at the wafer corresponding to the phase edge, with excellent contrast if it is isolated enough. With just one phase edge defining lines, it would be impossible to pattern arbitrary layouts without a second mask to clear unwanted phase edges. CPL allows one to avoid using a second mask by patterning narrow lines with two closely-spaced parallel phase edges that cannot be resolved. The combined aerial image of the two parallel phase edges is still a deep single minimum that patterns as one line but now the “line” on the reticle (mask) can be drawn just as it would with chrome, wherein the chrome is replaced by a phase shifted region. However, this only works for lines that are not wide; if the phase shifted line becomes too wide, i.e. the two phase edges of the line move too far apart, then they become individually resolvable and will pattern as two parallel lines. If the phase shifted line is too narrow, the aerial image contrast gets worse very quickly as the phase shifted region become smaller and looks more like a uniform piece of quartz. These two cliffs constrain the size of phase shift lines to a relatively tight range of small widths.
These effects are illustrated in the aerial image diagram of FIG. 1, which corresponds to a simulation with 193 nm light, a 0.68 NA (numeric aperture) projection lens, and quadrupole illumination (0.1 sigma poles at 0.7 sigma radii along line/space axes). The ideal case corresponds to a 0.1 μm separation, which produces a deep single minimum. As the separation width increases, the aerial image results in a pair of minimums being produced, as shown by the 0.2 μm and 0.5 μm separation curves. For example, a separation of 0.5 μm would result in two lines being resolved. This of course is undesired. As a result, wider lines are typically patterned using a binary (i.e., chrome-patterned) reticle.
A similar evolution occurs for isolated CPL contact aerial images, as shown in FIG. 2, which illustrates aerial image dependence on contact size for dense (0.20 μm pitch 2D grid) contact spacing. As the CD (critical dimension) increases, the minimum drops quickly and the normalized image log slope (NILS) rises. At some critical size, the minimum begins increasing and the maxima continue to decrease, causing the NILS to get worse to the point at which contact holes cannot be patterned.
Another widely recognized difficulty with CPL for line/space patterning arises when trying to pattern dense 1:1 (equal line and space widths) line/spaces. As the line pitch becomes tighter, the contrast of the aerial images of the lines quickly becomes worse until it is just a flat background, as shown by the solid-line curve of FIG. 3. This loss of contrast prevents CPL from being easily used to produce lines at tight pitches. This complete patterning failure for dense lines/spaces has been one reason that CPL has not been widely used in practice.